Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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0021 Distillation Steam distillation finds application in

the analyses of volatiles from beverages and high-

water-content foods, although it is less applicable to

fats and oils. It has the disadvantage that the large

quantities of aqueous distillate require further extrac-

tion with a solvent, to separate the volatiles from the

water. Concentration of the extract is then necessary.

The formation of artefacts may also be a problem.

Vacuum distillation finds wide application in the

extraction of volatiles from fats and oils, and at low

extraction temperatures artefact formation can be

minimized. In molecular distillation, a high vacuum

(< 10

3

mmHg), is used, and the volatiles have a rela-

tively short path to travel from the surface of a liquid

oil sample to a cold surface where they condense.

Efficient collection of high-boiling-point components

is achieved because at high vacuum, the distance

between the sample surface and the cold condenser

(10–20 mm) is less than the mean free path of the

molecules. All of these distillation processes incorpor-

ate cold traps for the collection of the distillate.

0022 Extraction Direct extraction of an aqueous food

fraction with an organic solvent is of limited use,

because the extract will contain a large amount of

nonvolatile matter. However, supercritical carbon di-

oxide has been utilized as a solvent for the extraction

of aromas on both a laboratory and a commercial

scale, particularly the preparation of high-quality

essential oils. Its solvating qualities can be altered

by changing the pressure or temperature at which

the extraction takes place, and under ideal condi-

tions, supercritical carbon dioxide exhibits a strong

affinity for most aroma compounds, whereas most

nonvolatile constituents are insoluble. The ease of

removal of the solvent, after extraction, to give a

concentrated aroma extract is another attractive fea-

ture of supercritical carbon dioxide extraction.

0023 One of the most widely used techniques in aroma

analysis combines steam distillation with solvent ex-

traction in a Likens–Nickerson apparatus (Figure 3),

which was first reported in 1964 for the extraction of

hop oil. The essential feature is the simultaneous

condensation of the steam distillate with an immis-

cible extraction solvent. A simple U-tube, with appro-

priate side arms, allows the return of water to the

steam distillation flask and the solvent, containing

the extracted volatiles, to a reservoir flask where it

redistils for further extraction. The technique pro-

vides a simple rapid method involving only small

volumes of solvent. By carrying out the extraction

under a reduced pressure, thermal degradation of

labile components can be diminished.

0024 The organic solvents used for extraction are chosen

on the basis of their selectivity for the volatile

compounds of interest and on boiling point. Com-

monly used solvents are diethyl ether, pentane,

isopentane, dichloromethane, and some chloro-

fluorocarbons. After each type of solvent extrac-

tion, most of the solvent must be removed to

provide an aroma concentrate suitable for GC.

Removal of traces of water, using a drying agent

(sodium or calcium sulfate) or by freezing out at

20



C, is essential before careful removal of solvent

by distillation. Final concentration to a volume of

0.1–1 ml is often achieved by purging with a gentle

stream of nitrogen.

Separation of Aroma Components

0025To determine the sensorially important compounds in

an aroma isolate, the complex mixture needs to be

separated into its components. The amount of isolate

is usually small, containing many compounds of di-

verse chemical structures, varying greatly in concen-

tration, and important components are often present

in extremely low amounts. The success of any aroma

analysis depends mainly upon the efficiency of

separation and the sensitivity of detection. GC using

bonded-phase fused-silica capillary columns is almost

universally used as the separation method in aroma

analysis. Such columns can separate complex mix-

tures, and the most commonly used stationary phases

are Carbowax 20M, a polar phase, and the two non-

polar phases, 100% poly(dimethylsiloxane) and

poly(5% diphenylsiloxane/95% dimethylsiloxane).

The stereochemistry of compounds can influence

water bath

heating mantle

flow of solvent

vapour

flow of steam

cooling

water at 5 C

solvent

sample in

water

fig0003Figure 3 Likens–Nickerson apparatus for continuous steam

distillation–solvent extraction.

SENSORY EVALUATION/Aroma 5177

aroma, e.g., (þ)-carvone possesses a spearmint aroma,

whereas ()-carvone possesses a caraway-like aroma;

capillary columns coated with chiral phases can be

used to separate enantiomers. The retention times of

an aroma compound on two columns with different

stationary phases can be helpful in its identification,

and databases containing retention data for volatile

compounds are available.

0026 Preparative liquid chromatography is sometimes

performed on solvent extracts, as a first step, to

simplify subsequent separations. The most widely

used example of this is flash chromatography, where

the extract is pipetted on to the surface of a stationary

phase, such as silica gel, which is contained in a glass

column. Fractions of the extract are obtained, as

solvents of increasing polarity are passed through

the column under positive air pressure. The fractions

are analyzed subsequently by GC. (See Chromatog-

raphy: Gas Chromatography.)

Detection of Components of Sensory Significance

0027 An essential step in the analysis of aroma is to deter-

mine which volatile compounds contribute to the

aroma of the food or beverage. Sensory profiling of

the whole product can give some indication of the

important aroma characters present in the product,

and such information should be used when assessing

the contribution of the individual components of an

aroma isolate to the overall flavor quality. A widely

used technique for determining components that

contribute to aroma is GC olfactometry (GCO). The

column effluent is split between a conventional GC

detector and a vent to the outside of the oven, where

the odors emerging can be smelled and described.

Chromatogram peaks can thus be identified that cor-

respond to specific aromas, and ‘aromagrams’ can be

compiled to complement the chromatogram.

0028 To estimate the relative contributions of volatile

components towards the total aroma quality of a

food, a set of serial dilutions of an aroma extract

can be made up and analyzed by GCO. At a certain

dilution, the aroma of a particular volatile compound

will no longer be perceived. The flavor dilution factor

for that compound is defined as the highest dilution at

which that compound can be perceived by GCO. For

example, if the concentration of the extract was

halved at each dilution and the seventh dilution was

the last at which the compound could be detected, its

flavor dilution factor would be 2

(128). Hence, if the

aroma extract is representative of the food from

which it is derived, the most important contributors

to the aroma of the food are those with the highest

flavor dilution factors. It should be noted that com-

ponents with high flavor dilution factors might not

give GC peaks of any significant size.

0029Another way of estimating the contribution of a

volatile compound to the aroma of a food is by calcu-

lating its odor unit number. The odor unit number of

an aroma compound in a food is defined as the ratio

between its concentration in the food and its odor

threshold, as measured by sensory evaluation. Hence,

the higher the odor unit number of the compound, the

more likely it is to contribute to the overall aroma of

the food. Several literature compilations of odor

thresholds exist.

Identification of Components

0030Structure elucidation of the chromatographically sep-

arated components is the next step in the analysis of

an aroma isolate. Coupled gas chromatography–mass

spectrometry (GC–MS) allows direct analysis of the

separated components and provides the most efficient

means of volatile identification.

0031Gas chromatography–mass spectrometry The avail-

ability of coupled gas chromatograph–mass spec-

trometer systems in the late 1960s was one of the

most important developments for flavor research,

and this was followed a decade later by the introduc-

tion of efficient computerized data-handling systems.

Today, sensitivity is usually such that a full spectrum

can be readily obtained on 1 ng of a single component

in a complex mixture injected on to the GC column,

and in many cases, an identifiable spectrum can be

obtained from as little as 10 pg. Quadrupole mass

spectrometers, obtaining at least one spectrum per

second, are ideal for low-resolution GC–MS of

aroma extracts. However, double-focusing magnetic

sector mass spectrometers can acquire accurate mass

data, allowing the calculation of empirical formulae

for ions in a spectrum, which can be a great asset in

the identification of unknown compounds.

0032The characterization of unknown compounds is

greatly facilitated by comparing their mass spectra

with those of known compounds in compiled libraries

incorporated into the GC–MS data system. Current

databases contain the mass spectra of over 200 000

compounds, but special compilations containing only

volatile compounds have also been prepared, and at

least one is available in computer-readable format.

Confirmation of the identity of compounds should

always be carried out, preferably by comparing their

mass spectra and retention times with those of

authentic samples. (See Mass Spectrometry: Prin-

ciples and Instrumentation.)

0033Spectroscopic methods Nuclear magnetic resonance

and infrared (IR) spectroscopy are both extremely

valuable techniques in the identification of organic

compounds. However, they require larger sample

5178 SENSORY EVALUATION/Aroma

sizes than mass spectrometry and are not as widely

used in aroma analysis. They are primarily used to aid

structure elucidation of unknown compounds.

0034 Fourier-transform IR instruments directly coupled

to a gas chromatograph have been developed, giving

adequate vapor-phase spectra on 10–100 ng of com-

pound. IR offers functional group information and

in general is complementary and supportive of MS.

Because IR is nondestructive, it has been possible to

develop tandem GC–IR–MS systems, providing a

powerful tool for aroma analysis.

Quantification of Aroma Components

0035 Often, quantitative information on aroma com-

pounds in a food is needed, such as for the calculation

of odor unit numbers. Quantification is rarely simple,

because most extraction techniques only remove a

proportion of the aroma from the food. Sometimes,

the extraction technique may generate aroma com-

pounds, and difficulties in quantification may arise

when compounds are not resolved by GC. The most

effective means of quantification is isotope dilution

assay using GC–MS, where a known amount of a

C- or

H-labeled internal standard is added to a

slurry of the food to be extracted, in order to quantify

its nonlabeled equivalent. As the labeled and un-

labeled aroma compounds possess similar physical

properties, the proportion of each extracted will be

the same. The relationship between the labeled stand-

ard and the compound of interest can be used to

calculate accurately the amount of the compound of

interest in the food.

0036 Other quantification methods include the addition

of an internal standard, not present in the food, of

a similar chemical composition to the compound

of interest, e.g., 2-methylpentanal could be used to

quantify hexanal. Conversely, standards could be

added to the extract, in order to calculate the amount

of the compound of interest in the extract rather than

the food itself. Standards can be injected into traps

containing adsorbent and, in the case of SPME,

injected on to the GC column immediately before

desorption of the fiber.

The Electronic Nose

0037 The electronic nose is an array of chemical sensors,

connected to a pattern-recognition system that

responds to odors passing over it. Different odors

cause different responses in the sensors, and these

responses provide a signal pattern characteristic of a

particular aroma. The computer evaluates the signal

pattern and can compare the aromas of different

samples, using pattern recognition. For example, if a

manufacturer uses coffees from around the world, the

electronic nose can analyze them, and their aromas

can be plotted on a multidimensional response map.

The analysis of several samples of each coffee, under

ideal conditions, can result in a group of points for

each coffee, well resolved from any other group. As

the number of samples increases, the resolution be-

tween the groups should also increase. Plotting the

sensor responses of an unknown sample on the same

map should permit its recognition, by its proximity to

one of the known samples.

0038Sensors are usually made of metal oxides or

organic polymers, although more recently, surface

acoustic waves and piezoelectric crystals have been

used. Problems may exist when samples with a high

water content are analyzed, as many of the sensors

respond strongly towards water, preventing any

sample differences being observed.

Measurement of Aroma Release during

Eating

0039Attempts have been made by several researchers to

measure the release of aroma during the eating of

food. At present, mass spectrometry using an atmos-

pheric pressure chemical ionization (APCI) interface

appears to be the most effective way of doing this. As

a subject eats a food or beverage, a sampling tube

draws the volatiles from the mouth or nose into the

mass spectrometer interface. Released aroma com-

pounds are ionized in air containing water vapor at

atmospheric pressure in the APCI source; the ions are

subsequently sampled into the high-vacuum area of

the mass spectrometer. APCI does not normally

induce fragmentation, and a strong protonated mo-

lecular ion is observed. By monitoring the molecular

ions associated with the major aroma compounds in

the sample, volatile release profiles can be measured.

Pulsed release curves are usually obtained, due to

respiration of the subject, and these can be smoothed

to show how each of the volatiles of interest behaves

during the eating process. This equipment has been

used, for example, to examine how reducing the fat

content of a food will affect its flavor or to study how

the chewing patterns of different people affect flavor

release.

See also: Chromatography: Gas Chromatography;

Combined Chromatography and Mass Spectrometry;

Essential Oils: Properties and Uses; Flavor (Flavour)

Compounds: Structures and Characteristics; Food

Acceptability: Affective Methods; Mass Spectrometry:

Principles and Instrumentation; Applications;

Spectroscopy: Infrared and Raman; Nuclear Magnetic

Resonance; Taints: Types and Causes; Analysis and

Identification

SENSORY EVALUATION/Aroma 5179

Further Reading

Acree TE and Teranishi R (eds) (1993) Flavor Science:

Sensible Principles and Techniques. Washington, DC:

American Chemical Society.

Goodenough PW (1998) The molecular biology of olfac-

tory perception. International Journal of Food Science

and Technology 33: 63–77.

Maarse H (ed.) (1991) Volatile Compounds in Foods and

Beverages. New York: Marcel Dekker.

Marsili R (ed.) (1997) Techniques for Analysis of Food

Aroma. New York: Marcel Dekker.

Meilgaard M, Civille GV and Carr BT (eds) (1991) Sensory

Evaluation Techniques, 2nd edn. Boca Raton, FL: CRC

Press.

Morello M and Mussinan C (eds) (1998) Challenges in the

Extraction and Isolation of Food Flavors. Washington,

DC: American Chemical Society.

Nijssen LM, Visscher CA, Maarse H, Willemsens LC and

Boelens MH (1996) Volatile Compounds in Food –

Qualitative and Quantitative Data, 7th edn. Zeist, The

Netherlands: TNO Nutrition and Food Research

Institute.

Pawliszyn J (ed.) (1999) Applications of Solid Phase Micro-

extraction. Cambridge: Royal Society of Chemistry.

Taylor AJ and Mottram DS (1996) Flavour Science: Recent

Developments. Cambridge: Royal Society of Chemistry.

Taste

H Valentova

and Z Panovska

, Institute of Chemical

Technology, Prague, Czech Republic

Taste

0001 Taste, according to the International Standard Sens-

ory Analysis Vocabulary ISO 5492 (1992), is defined

as sensations perceived by the taste organ when

stimulated by certain soluble substances. Taste is

closely related to smell. The perception of odor and

taste, combined with trigeminal sensations, results in

the overall flavor. Flavor influences food acceptance

and selection of food intake, and helps us to

distinguish potentially harmful compounds. The

taste sensation is a very complex process starting at

the sensory receptor level and finishing in the central

nervous system, where it is combined with informa-

tion coming from other senses. The earliest work

concentrated on anatomical studies, and so we have

a good deal of information about the structural taste

system on the tongue.

Anatomy of the Tongue

Mouth

0002An important role in the perception of taste is played

by our mouth cavity and especially by the tongue.

Food is placed inside the mouth, where our teeth

chew it, and the saliva makes it into a solution, and

therefore, the food comes into contact with the sur-

face of the tongue. The main role for perception of

taste is played by taste papillae, taste buds, and taste

cells.

Papillae

0003Taste papillae are located on the tongue, soft and hard

palate, pharynx, and epiglottis. The papillae give the

tongue its bumpy appearance and can be classified,

according to their shape, into four types: fungiform,

foliate, circumvallate, and filiform papillae. The first

papillae appear when the human fetus is 6 weeks old,

and at around 10 weeks, the first primitive taste pores

are found. Table 1 shows a description of the different

types of papillae.

Taste Buds

0004In the epithelia of the papillae, there are taste buds,

40–80 mm in size. Children have about 10 000 taste

buds, but the amount decreases with age. The average

number for adults is 2000–3000 buds. The taste-bud

density and placement of the taste buds among people

vary. For example, the density on the tongue tip

ranges from 3–512 taste buds per square centimeter,

with an average of around 115 buds per square centi-

meter. People with a higher density of buds perceive

taste stimuli more intensively. The specificity of indi-

vidual taste bud cells is being studied. It is still not

clear at the taste bud level whether there is modality

specificity. Some studies have confirmed that some

fungiform papillae contain only one taste bud, but

this could not be proved. It has also been demon-

strated that taste buds respond differently to different

tbl0001Table 1 Descriptionofpapillae

Types of papillae Number of

papillae

Taste buds

perpapilla

Location

Fungiform 100–400 0–22 Base and edges of

the tongue

Foliate 8–20 10 Each side of lingual

posterior

Circumvallate 3–13 100–250 V-shaped pattern

across the root of

the tongue

Filiform 1000 No taste buds All the tongue

Average values are given because numbers differ among authors.

5180 SENSORY EVALUATION/Taste

substances. It seems that all areas with taste buds

respond to some degree to all tastes, but there is

usually one taste to which the response is the best.

Taste Cells

0005 The tip of the taste bud has a small opening about

2–5 mm in diameter, called a taste pore, where taste

stimuli interact with taste receptors. Taste buds are

aggregates of some 40–60 elongated epithelial cells.

There are different types of cells in a taste bud: dark

cells, light cells, cells with dark-cored and synaptic

vesicles, basal cells, and perigemmal cells. It is not

clear which of the cells serve as receptor cells or if the

different cell types are specialized to detect different

ranges of the gustatory spectrum. The taste cells con-

tinually regenerate; they live for only 7–10 days and,

within this period, grow to maturity and die. The

young taste receptor cells move from the periphery

of the taste bud to its center, and the older taste cells

in the central portion of the taste buds die away and

are then pressed out from the taste bud into the

mouth through taste pores. By replacing the cells in

this way, the gustatory system is protected from

losing its sensitivity. All taste cells from one taste

bud are connected by basal synapses to about 50

afferent nerve fibers. The nerve fibers from taste

buds converge into different cranial nerves.

Cranial Nerves

0006 The sense of taste is mediated by three cranial nerves.

The anterior (front) two-thirds of the tongue and

palate is supplied by branches of facial nerves, namely

chorda tympani branch (seventh cranial nerves), the

back of the tongue by glossopharyngeal nerve (ninth

cranial nerves), the base and central region, and the

throat and larynx by certain branches of vagus nerve

(tenth cranial). All nerves subserve not only taste but

also touch, temperature, and pain sensitivity. The

lingual nerve is the branch of the fifth cranial nerve

and is responsible for the perception of pungency,

heat, cooling, and others.

0007 The taste fibers from all the sensory nerves from

the mouth come together in the brainstem (medulla

oblongata). From the brain’s medulla, the gustatory

fibers ascend by a pathway to a small cluster of cells

in the thalamus and thence to a taste-receiving area in

the anterior cerebral cortex.

0008 Two main theories try to describe how taste infor-

mation is coded in the brain. In the pattern theory, the

main role is played by taste receptor cells that respond

with different sensitivities to the different tastants.

In the labeled line theory, the four tastes are sensed

in four discrete type of receptor cells, and the infor-

mation from receptors goes to the brain through

independent channels. It seems that both types of

mechanism are involved in taste perception.

Transmission of Taste Through Nerve

Impulses

0009The transmission of electrochemical impulse through

the nerves is called transduction. The understanding

of this process has been the main focus of taste

researchers.

0010Generally, the arrival of a chemical stimulant on

the surface of a receptor changes the ionic conduct-

ance in the taste membrane and the electrochemical

impulse is produced. The transduction pathway in-

volves a number of different receptors and different

cellular mechanisms.

0011Taste stimuli may interact:

0012directly with the ion channel – this produces recep-

tor cell depolarization either by direct permeation

or by blockage of the ion channel;

0013with receptors and alter the flux of ion through the

ion channel (K

,Na

,Ca

);

0014may diffuse through lipid phase and affect G pro-

tein or the endoplasmatic reticulum.

Researchers believe that each basic taste has a par-

ticular transduction pathway (Table 2) that varies

with the location of taste buds on the tongue. For

tbl0002Table 2 Suggested pathways for individual tastes

Taste Possible pathways for transduction mechanism for different

stimuli

Bitter Multiple mechanisms – depending on the structure of

the compounds:

(a) Inhibition of apical K

channel (mechanism for

chinine, divalent salts)

(b) Blockage of Na

amino transport system

phospholipases

Sweet Specific membrane receptors are required for

transduction of sugars:

(a) Multiple sweet receptors coupled to G protein

(gastducin) and second messenger system

(b) Stimulus-gated ion channel

Sour Does not require specific taste receptor – several

apically located ion channels may participate in sour

transduction:

(a) sodium ion channel – proton enters taste cells

through amiloride-sensitive sodium channels;

(b) channels are stimulated by protons that permit

entry of cations into the cell

Salty Does not require specific taste receptor – direct

permeation of Na

ions through amiloride-sensitive

sodium channel

Umami Stimulus-gated ion channel: taste stimulus binds to a

receptor site on a receptor–ion channel complex

SENSORY EVALUATION/Taste 5181

sweet and bitter taste, there probably exist multiple

pathways that can be common for substances with a

similar structure. The transduction of the different

taste stimuli has been studied on nonhuman species,

and it is not still clear whether the same mechanism is

involved in humans.

Basic Tastes

0015 Over the centuries, the numbers of primary tastes

have varied from two – sweet and bitter – to more

then 10. Studies carried out to date have distinguished

four taste categories: sweet, salty, bitter, and sour

taste. Now, some of the scientists also include be-

tween basic tastes a less-established taste, ‘umami.’

International Standards ISO 5492 in paragraph 3.22,

under basic taste, states: ‘basic taste: any one of the

distinctive tastes: acid, bitter, salty, sweet, alkaline,

umami, metallic.’

Sweet Taste

0016 Sweet taste is one of the favorite tastes and therefore

the most studied. There is evidence that prenatal

infants detect and like sweet in utero and that

human infants are born with a preference for sweet

taste.

0017 Scientists tried to solve the problem of sweet taste

through the chemical structure of sweet compounds,

but substances that have a sweet taste are found in all

chemical classes. They are usually associated with

multiple hydroxyl groups, and a- but not b-org-

amino acids. Salts such as beryllium or lead salts,

simple organic compounds such as chloroform, and

alcohols can have a sweet taste. The main groups

are carbohydrates, with sweetness decreasing in a

homologous series sugars > oligosaccharides > poly-

saccharides. Other sweet organic compounds include

amino acids, peptides, proteins, compounds with

groups such as amides, ureas, and hydrazides.

Benzene compounds with one single NO

group are

often sweet, and those with more NO

groups

are usually bitter. Small changes in the stereochemie

of atoms within a molecule may affect its taste. The

chemical structure of the sweet compound also plays

an important role in the determination of the area of

the tongue, where the sweet taste is perceived. For

example, the taste of inorganic salts and nitrogen

compounds is perceived on the front of the tongue,

some dihydrochalcons on the back, and some sweet

proteins such as thaumatine and nonelline on the

sides.

0018 From the time when, in 1914, the need for discrete

functional groups of a sweet compound was recog-

nized, several theories tried to explain the sweet taste.

In Shallenberger’s hypothesis sugar molecules must

possess more hydrogen-bonded functional groups

per carbon. The molecules interact with the sweet-

taste receptor sites of a protein through hydrogen

bonds. The question is if there is a receptor for

sweet taste or if a different structural class has its

own special receptor and how they interact with the

sweet compound. It seems that this question will be

answered soon. Recently, scientists discovered a

sweet receptor gene, T1R3, thought to be the likely

basis for the tongue’s sweetness sensor. One of the

newest theories is multipoint attachment theory. This

theory assumes the presence, probably a seven-pass

transmembrane receptor, of at least eight fundamen-

tal recognition sites. The multiple theory is supported

also by other findings, such as synergy effects of

sweeteners, the adaptation of the response to some

sweeteners, but not to others, the sensitivity of differ-

ent parts of the tongue to the different sweeteners,

and human variability in sensitivity to different

sweeteners.

0019Studies with a new approach to studying the mech-

anism of sweet taste also appeared during the last

decade. Schallenberger in 1993 described the role of

symmetry. Sweetness is elicited through a bilaterally

symmetrical and concentrated dipolar interaction

between glycophore and the receptor. Also, the role

of partial molar and specific volumes has been ex-

plored in sweet taste chemoreception. Some scientists

focused on the role of water that could be the vehicle

by which stimulus molecules are transported to the

receptor environment and oriented towards it. Recent

studies suggest that there is a different transduction

mechanism for natural sugars and synthetic sweeten-

ers that could be activated by different G proteins

from those activated by sucrose. Although the sweet

taste may be present in hundreds of compounds,

fewer than 30 are permitted for use. The sensory

reference standard for sweet taste is sucrose. No

other sweetener is perceived to be identical; for

example, glucose produces moderate burning and a

bitter taste. Humans are more sensitive to synthetic

sweeteners. A comparison of the sweet taste of

synthetic sweeteners with that of sucrose is given in

Table 3. Some synthetic sweeteners have a bitter and

metallic taste, so they are used in mixtures.

Bitter Taste

0020Bitter taste is closely linked to sweet taste. Some

compounds can have both a sweet and bitter taste,

and slight structural modifications can alter their

intensities. Bitter compounds require polar (electro-

philic or nucleophilic) and hydrophobic groups and

must be at least slightly soluble in water. Among

the best-known bitter substances are alkaloids

(often toxic) such as quinine, caffeine, and strychnine.

5182 SENSORY EVALUATION/Taste

Alkaloids are often found in many plants, and so

bitter taste is thought to be a mechanism for poison

avoidance by animals. Also, salts that contain the

cesium or rubidium cation or the iodide anion are

bitter. Other bitter-tasting compounds include pep-

tides, amino acids, ureas, thioureas, terpenoids, and

polyphenols. The ability to perceive bitter substances

has been related to their lipid solubility, which can be

altered by pH. Bitterness is perceived towards the

back of the tongue, but some compounds are per-

ceived also on the tip. Transduction of bitter tastants

can involve multiple mechanisms, depending on the

structure of the bitter compound. One group of bitter

compounds may be detected by several different

mechanisms. The existence of more bitter receptors

is confirmed by a single Mendelian gene, which

affects the ability to taste one particular class of bitter

compounds. Recently, remarkable progress has been

made in establishing the nature of bitter taste. Two

groups of scientists have confirmed the existence of

bitter taste receptors. They identified a large family of

G-protein-linked receptors (40–80 members). These

receptors are organized in clusters.

0021 One of the most interesting bitter compounds that

has been studied in the past decade is 6-n-propyl-

2-thiouracil (PROP). Individuals detect this com-

pound with very different sensitivities that could be

inherited.

Salty Taste

0022 Salty taste is the last of the basic tastes that a newborn

child perceives (at approximately 4 months). It is

mainly associated with the taste of NaCl. At first, it

was thought that the saltiness of NaCl is caused by



anions, but later, scientists preferred the theory

that saltiness is produced by Na

cations and that

anions modulate the intensity of the saltiness (larger

anions reducing saltiness more than smaller anions).

The molecular weight of the cation does not influence

the saltiness, but anions with a low molecular weight

are predominantly salty, whereas those of a higher

molecular weight tend to be bitter. The salts of

heavy metals such as mercury have a metallic taste.

The degree of saltiness decreases in the following

order: ammonium (most salty), potassium, calcium,

sodium, lithium, and magnesium salts (least salty).

This order can vary for different animals. Some salts

in low concentration can taste sweet, NaCl tastes

sweet in concentrations below 0.04 M, KCl below

0.02 M, and in some salts, especially lead acetate

and beryllium chloride, the sweet taste dominates. It

was found that people are more sensitive to NaCl at

the tip than on the more posterior region of the

tongue and that NaCl taste thresholds are system-

atically related to the size and region of tongue

stimulated.

0023Although sodium chloride is essential for life and is

the second most-used additive, it is believed to

contribute to hypertension and some cardiovascular

diseases. Therefore, scientists are looking for possible

substitutes. Other chlorides have a salty taste but

cannot be used as a good substitute because of their

bitter and sour taste or toxic effects. Recently, pep-

tides with a salty taste (l-ornithyltaurine, glycine

methyl, or ethylester hydrochloride) and also differ-

ent dipeptides were examined. The food industry tries

to avoid using salt by replacing it with herbs, spices,

and organic acids.

Sour Taste

0024Sour taste is acceptable when mild but becomes un-

pleasant when strong. It helps us to avoid unripe fruit

and damage our tissue with acids. In humans, acids

induce salivation, which increases bicarbonate secre-

tions, the major buffering agent in saliva.

0025From a chemical point of view, acids can be distin-

guished by their pH index (describes the activity of

protons in a solution) as a weak pH 3–7 and strong

pH 1–3. We cannot use this distinction when we look

at sour taste. Weak organic acids (e.g., the acetic acid

in vinegar) taste more sour than mineral acid at the

same pH.

0026The perception of sour taste is influenced not only

by the activity of the proton, but also by the quality

and character of the anion. The role of anion has not

been studied in detail at receptor level. Sour taste in

food is mainly connected with citric, malic, oxalic,

and tartaric acids in fruits and lactic acid in yogurt

and other dairy and meat products. Propionic acid is

found in Emmental cheese, and acetic acid is used in

the canning industry.

Umami

0027In 1909, the Japanese scientist Ikeda described a

special delicious or savory taste and used the Japanese

word ‘umami.’ This taste is familiar in the East, where

seaweed and mushroom are used as ingredients,

tbl0003 Table 3 Relative sweetnesses for several artificial sweeteners

compared with 3% sucrose solution

Sweetener Date of discovery Relative sweetness (sucrose ¼1)

Saccharin 1879 300–500

Dulcin 1883 70–350

Cyclamate 1937 30–60

Aspartame 1965 100–200

Acesulfame-K 1967 130–200

Sucralose 1974 600

Alitame 1979 2900

SENSORY EVALUATION/Taste 5183

because they have a high content of glutamate. This

taste is sometimes called a savory or meat-like taste,

because meat also has a high content of glutamate.

Extensive scientific tests carried out in the twentieth

century indicated that the umami taste is different

from the four classical tastes and that it is connected

with the l form of monosodium glutamate (MSG).

Later, nucleotides such as guanosine-5

-monopho-

sphate (GMP) and inosine-5

-monophosphate (IMP)

were also considered to have umami flavor.

0028 The occurrence of peptides (around 30), usually

dipeptides and tripeptides, with a possible umami

taste has also been reported, but this has not been

confirmed by other scientists. Some dipeptides have a

salty or bitter taste, or their taste is described as a

salty/umami or sour/umami taste.

0029 Faurion in 1991 found that the MSG taste is recog-

nized by more than one type of glutamate receptor.

Recently, the genes for proteins that serve as the

molecular receptors for umami taste were identified.

Other Tastes

0030 Other tastes such as metallic, soapy, alkaline, and

astringent are described for the use of sensory evalu-

ation but have not been studied at the receptors level.

0031 Astringency taste Astringency is a tactile taste felt as

a dry, rough feeling in the mouth and contraction of

the tongue tissue. It usually involves the formation of

aggregated precipitates between tannins or poly-

phenols and proteins in the saliva. Tannins positively

influence the taste of tea, coffee, cocoa, wine, and

beer but have a negative effect in immature fruit

(bananas). Lawless and co-workers developed a vo-

cabulary for astringency taste to describe its multiple

perception characteristics. They defined astringency

as a combination of three tastes: drying (the lack

of lubrication or moistness), roughing (the rough

texture in the mouth) and puckery (the drawing or

tightening sensation). There are also specific terms

such as sappy, harsh, woody, powdery, and gritty

that are used for describing astringency taste in wine

and beer.

0032 Metallic taste Metallic taste is elicited by some

metal salts, as a standard is used iron (II) sulfate

heptahydrate in concentration 0.00475 g l

1

. The

taste threshold for copper (using copper sulfate and

copper chloride) in different types of drinking water is

2.4–3.8 mg l

1

, depending on the type of water.

0033 Alkaline taste An alkaline, soapy taste is associated

with potassium carbonate (potash), a substance that

was used for the production of soap.

0034Pungency The trigeminal nerve is stimulated by

chemical-induced sensations such as pungency. A

pungent taste is characterized by spices such as hot

pepper (in which the pungency substance is the alkal-

oid piperine), ginger, Spanish red pepper (capsaicine),

and clove (eugenol). Vegetables from the Brassicaceae

family such as horse-radish, black and white mustard,

and cabbage have a pungent taste owing to their

isothiocyanates content. Pungency is also the main

sensory attribute of benzoic acid (3–36 mM). Some-

times, other sensations are described, such as the bite

of carbon dioxide, the sting of horseradish, and other

sensations derived from spices.

Factors Influencing Perception of Taste

Color

0035Several studies have examined the effect of color on

taste perception. The results depend on conditions,

concentration of taste substances, and intensity of

color. Generally, red and orange increase the thresh-

old for bitter, sour, and sweet taste, and green and

yellow increase the threshold for sour. The effect of

color on salty taste has not been studied.

Viscosity

0036The influence of viscosity has been studied mainly for

sweet compounds, but it is clear that it depends on the

taste substances and thickening agents. Viscous solu-

tions can suppress taste because of a lower diffusion

rate and poorer access to the receptors, but they can

enhance the taste because the time during which the

compounds bind to the receptors is longer. For

example, in a chocolate drink, sweetness increases

with increasing concentrations of lecithin. Usually,

the sweetness of sucrose is lower in liquid than in

aqueous solutions. The threshold of pure substances

in aqueous solutions is different from the threshold

values in other liquids.

Temperature

0037Temperature effects must be studied separately for

every taste and in a wide range of temperatures.

Sweet and bitter tastes are more affected by tempera-

ture than salty or sour tastes. Increasing the tempera-

ture usually increases the sweetness of the solution.

For some compounds, a V-shaped function has been

described, meaning that the minimum is near the

center of the temperature range. For example, the

sensitivity for NaCl was greatest in the range of

22–37



C and lower at 0 and 55



C. The optimum

temperature for the perception of bitter taste is



C, and the bitter aftertaste is longer than for

5184 SENSORY EVALUATION/Taste

other tastes. The sweetness of sugars is reduced in

cool solutions and the optimum is 35–50



Odors

0038 Smell and taste are closely related. Therefore, sev-

eral studies have concentrated on the relation-

ship between perception of an odor and the taste

of the solution to which the odor is added. It has

been confirmed that flavorings added to a solution

of sucrose increase or suppress the perception of

sweet taste. For example, the flavor of caramel, mar-

ajuca, and strawberry enhance sweetness, whereas

angelica oil and damascone suppress sweetness.

Also, caramel odor suppresses the sourness of citric

acid.

Interactions Between Basic Compounds

0039 Food is a complex of chemical substances, and there-

fore, it is important to study not only the individual

responses of the basic taste but also the responses and

interactions in a mixture. Although this problem has

been studied extensively and in a previous review, it is

difficult to summarize general conclusions. Usually,

only binary mixtures in aqueous solutions are stud-

ied, and their results cannot be extrapolated directly

to food.

0040 Moreover, the interaction between specific chem-

icals in a binary mixture must be studied also, because

compounds from one taste group, such as bitter

compounds, do not behave in the same way. Also, all

interactions must be studied over different concen-

tration ranges. An important role is played not only

by the type of mixture, but also by the type of experi-

mental work and differences in human sensitivities.

The interactions in mixtures can be linear – suppres-

sion (perceived as less intensive), enhancement (per-

ceived as more intensive) – and non-linear – masking

and synergism.

0041 The results can be summarized as follows:

0042 salts and acids enhance each other at moderate

concentrations but suppress each other at higher

concentrations;

0043 some salts enhance the sweet taste (potassium acet-

ate, potassium chloride, sodium chloride);

0044 sour substances inhibit sweetness and vice versa;

0045 bitter compounds and acids can either enhance or

suppress each other, depending on the concentra-

tion;

0046 bitter taste is generally suppressed by sodium salts,

and saltiness is unaffected;

0047 bitter and sour tastes are suppressed by nucleo-

tides, and salty and sweet tastes are enhanced by

nucleotides;

0048umami substances in water solution do not influ-

ence basic taste: there is only synergism between

umami substances.

Examples of Some Interesting Compounds

0049Caffeine and other methylxanthines can enhance

some sweeteners with a bitter component (acesul-

fam-K, thaumatin, steviosid, sodium saccharin), but

not compounds such as aspartame, sucrose, fructose,

and calcium cyclamate.

0050Certain amino acids, including d-phenylalanine, l-

and d-histidine, and l-arginine are also enhanced by

caffeine. Sweetness is inhibited, for example, by

sodium (þ)-2-(methylphenoxy)phenoxy propionate,

triterpene saponins (gymnemic acid, holodulcin, zizi-

phens), cyclohexyl acetic acid, indole acetic acid, and

a-naphthol sulfate.

0051Molecules that inhibit more than one taste are

called multisaphoric molecules. The glycoproteins

miraculin (without taste) and curculin (sweet taste)

have a special property, because they convert the taste

of acids from sour to sweet.

Cause of the Differences in Feeling Taste

0052There are more reasons why people differ in their

perception of taste. Some can be explained by differ-

ences in anatomy and psychology among people,

others by some illnesses or by the process of aging.

The most-studied factor that influences taste percep-

tion is aging. Among older people at about the age of

60, there is normally a decline in the sense of taste,

especially sour and bitter.

0053Disorders associated with taste are less common

than with smell, because taste is innervated by three

cranial nerves compared with a single cranial nerve

for olfaction. Ninety per cent of patients complaining

of a reduction or loss of taste had a reduction or loss

of smell. This was confirmed by findings that people

who had lost their sense of smell (anosmic) can dis-

tinguish between four basic tastes. Changes in the

sense of taste can be either temporary or permanent.

0054Decreased perception of taste is called hypogen-

eusia. This loss was initially thought to be due to a

reduced number of taste buds, but recent data indi-

cate that a decline in the number of receptors may be

responsible. A persistent taste in the mouth in the

absence of stimulation is called dysgeusia, and com-

plete loss of the sense of taste is called ageusia (gusta-

tory anesthesia).

0055Loss of the sense of taste can be due to acute and

subacute diseases: seasonal allergies, head injury,

acute viral illness, idiopathic causes, and also the

case of hormonal disorder, tumors in the brain,

cancer, disease of the gums or teeth, nerve damage,

SENSORY EVALUATION/Taste 5185

and so on. Burning mouth syndrome is an oral pain

disorder occurring primarily in women who have

difficulties with identifying salty and sour tastes.

The intake of certain drugs (for example anesthetics

such as pantocaine and novocaine) and exposure to

chemical substances can also influence our perception

of taste. For example, chlorhexidine, an antiseptic,

can decrease the bitterness of quinine, and it is the

only known blocker of the human salty taste.

0056 Some people cannot taste the bitter substance

phenyl thiocarbamide (taste blindness), and people

with congenital idiopathic hypoparathyroidism

cannot recognize sweetness.

Methods of Sensory Analysis

0057 The basis of sensory analysis is the perception of food

attributes through the direct use of human senses. The

main tool, therefore, is an assessor, who must be

selected and trained. There are standardizing methods

for training, but it is necessary to check, refresh

and update the performance of assessors. Sensory

methods must be standardized on international, re-

gional, or national scales. Of all the sensory methods,

the most useful for studying taste are detailed below.

Methods for Investigating Sensitivity of Taste

0058 Taste is studied by different methods of sensory

analysis that require good methodological and ex-

perimental control. The International Standard ISO

3972 describes the method of investigating sensitivity

of taste. First, the assessors identify the taste of

reference substances (for example, the concentration

for reference sucrose is 5.76 g l

1

, and for caffeine

0.195 g l

1

). It has been proven by practical tests

that these concentrations are detected by 50% of

novice assessors. Another approach is to determine

the threshold. We recognize three types: detection

threshold (stimulus threshold) is the minimum value

of a sensory stimulus needed to bring about a sensa-

tion. In a recognition threshold, accessors must iden-

tify this sensation, and the difference threshold is a

value of the smallest perceptible difference in the

physical intensity of a stimulus. The threshold values

from different studies differ between authors, but the

results also vary between individuals and the results

from one individual can vary, depending on their

health condition and mood, and so on. Among all

tastes, bitter generally has the lowest detection

threshold for naturally occurring compounds (see

Table 4). The threshold is usually measured by using

pure substances in aqueous solutions. The tempera-

ture of solutions and aqua used must be specified,

because there are differences between values obtained

from distilled water, tap water, and deionized water.

Time–intensity Technique (TI)

0059The time–intensity technique is the only technique to

measure changes in perceptual characteristics with

time. The idea of measuring sensation intensity ori-

ginated in the 1960s but now has become widely used

with the development of computerized systems for

data collection.

0060The concept of TI is founded on the assumption

that intensity and time are two independent dimen-

sions of sensory space. During measurement, curves

are produced, with time on the horizontal axis and

sensation on the vertical axis, and these curves can

indicate how long it takes to reach the maximum

intensity for that taste, the overall maximum

intensity, the total duration of sensation, the rate of

extinction, and the rate of appearance.

New Analytical Methods for

Determination of the Taste

0061Sensory analysis is a time-consuming and expensive

method that requires trained assessors or experts.

Attempts have been made to replace this method by

various chemical and instrumental methods. An ‘elec-

tronic tongue’ has recently been developed to meas-

ure taste (based on pulsed voltametry). The Japanese

electric tongue is composed of several lipid/polymer

membranes that transform the information of taste

substances into an electric signal.

0062Analytical methods can be useful but still are less

objective than sensory analyses, because the former

measure only stimuli and not sensations.

tbl0004 Table 4 Values of detection thresholds for individual tastes

Sweet Threshold

(mol dm

3

)

Salt Threshold

(mol dm

3

)

Sour Threshold

(mol dm

3

)

Bitter

Threshold

(mol dm

3

)

Saccharose 0.011–0.017 Sodium chloride 0.03 Citric acid 0.00078 Caffeine 0.00002–0.002

Lactose 0.065–0.072 Calcium chloride 0.017 Acetic acid 0.00183 Quinine 0.0000001–0.00001

Maltose 0.038 Ammonium chloride 0.004 Tartaric acid 0.00053 (þ)-Catechin 0.002

D-glucose 0.065–0.08 Sodium fluoride 0.005 Malic acid 0.00082 L-Leucine 0.015

Saccharin 0.000000023 Magnesium chloride 0.015 Hydrochloric acid 0.0078 Magnesium sulfate 0.005

A range is given because the values for bitter taste differ between authors.

5186 SENSORY EVALUATION/Taste